The present invention generally relates to head switching signal producing circuits, and more particularly to a head switching signal producing circuit for producing a head switching signal which is used to switch over reproduced signals from rotary heads of a helical scan type magnetic recording and reproducing apparatus so as to obtain a continuous reproduced signal which time-sequentially comprises the reproduced signals from the rotary heads.
In a helical scan type magnetic recording and reproducing apparatus (hereinafter referred to as a video tape recorder or simply VTR), a plurality of rotary heads successively scan a magnetic tape. Hence, in order to prevent noise from a rotary head which is not scanning the magnetic tape from mixing into a reproduced signal from a rotary head which is actually scanning the magnetic tape, the reproduced signals successively obtained from the rotary heads are supplied to a switching circuit and the switching of this switching circuit is controlled responsive to a head switching signal so that a continuous reproduced signal which time-sequentially comprises the reproduced signals from the rotary heads is produced from the switching circuit, as is well known. As methods of producing the head switching signal, there are basically two conventional methods.
According to a first conventional method, a rotational phase detector is used to detect a rotational phase of a rotary drum which is mounted with the rotary heads. Generally, the rotary drum is connected to a rotary shaft of a motor which rotates the rotary drum. According to a first conventional method, a rotary plate is fixed to the rotary shaft of the motor, and a number of magnets corresponding to the number of rotary heads are provided on the rotary plate. A magnetic head is located at the same height position as the rotary plate so that the magnetic head confronts the magnets on the rotary plate which rotates unitarily with the rotary drum. The magnetic head which constitutes a rotational phase detector together with the magnets on the rotary plate produces a pulse every time one of the magnets on the rotary plate passes the position confronting the magnetic head. Output pulses PG of the magnetic head, that is, the rotational phase detector, are supplied to a delay circuit wherein the timing of the pulses is adjusted to each of the rotary heads, and the head switching signal is produced from an output signal of the delay circuit.
However, according to the first conventional method, it is necessary to provide on the rotary plate a number of magnets corresponding to the number of rotary heads, and mounting positions of the magnets on the rotary plate must be adjusted with respect to the corresponding rotary heads on the rotary drum. As a result, there are problems in that it takes time to adjust the mounting positions of the magnets on the rotary plate with respect to the corresponding rotary heads with a high precision, and it is difficult to obtain an accurate head switching signal.
On the other hand, a second conventional method uses a frequency generator in addition to the rotational phase detector of the first conventional method. The frequency generator is coupled to the motor which rotates the rotary drum and generates pulses FG having a period dependent on a rotational speed of the rotary drum. Generally, the output pulses FG of the frequency generator are used to control the rotational speed of the rotary drum. The output pulses FG of the frequency generator are indicative of rotary positions of the rotary drum. Hence, it is possible to produce a head switching signal for each of the rotary heads by counting the output pulses FG of the frequency generator with reference to the output pulses PG of the rotational phase detector.
According to the second conventional method, an N-count counter counts the output pulses FG of the frequency generator, and this counter is reset by the output pulses PG of the rotational phase detector indicative of a reference phase. It is possible to detect the rotational phase of the rotary drum from a counted value in the counter, but problems occur depending on timings of the output pulses PG of the rotational phase detector and the output pulses PG of the frequency generator. For example, when the timing with which the pulses FG are counted in the counter and the timing with which the counter is reset by the pulses PG coincide or approximately coincide and the timings with which the pulses FG and pG are supplied to the counter become inverted due to change with time or the like, a timing error of one period of the pulses FG occurs in the head switching signal because the head switching signal is produced when the counted value in the counter reaches a predetermined value. For this reason, the mounting positions of the rotational phase detector and the frequency generator with respect to the rotary heads are appropriately controlled so that the timing error does not occur.
The control of the mounting positions of the rotational phase detector and the frequency generator is relatively easy when the number of pulses FG generated from the frequency generator in one revolution of the rotary drum is small. However, when the number of pulses FG generated from the frequency generator in one revolution of the rotary drum is made large in order to improve the accuracy of the rotational speed control described before, one pulse out of the pulses FG corresponds to a small rotary angle of the rotary drum, and there is a problem in that it is extremely difficult to mount the rotational phase detector within a tolerance of such a small rotary angle.